.net - Why doesn't C# support implied generic types on class constructors?

Question

C# doesn't require you to specify a generic type parameter if the compiler can infer it, for instance:

List<int> myInts = new List<int> {0,1,1,
    2,3,5,8,13,21,34,55,89,144,233,377,
    610,987,1597,2584,4181,6765};

//this statement is clunky
List<string> myStrings = myInts.
    Select<int,string>( i => i.ToString() ).
    ToList<string>();

//the type is inferred from the lambda expression
//the compiler knows that it's taking an int and 
//returning a string
List<string> myStrings = myInts.
    Select( i => i.ToString() ).
    ToList();

This is needed for anonymous types where you don't know what the type parameter would be (in intellisense it shows up as 'a) because it's added by the compiler.

Class-level type parameters don't let you do this:

//sample generic class
public class GenericDemo<T> 
{
    public GenericDemo ( T value ) 
    {
        GenericTypedProperty = value;
    }

    public T GenericTypedProperty {get; set;}
}

//why can't I do:
int anIntValue = 4181;
var item = new GenericDemo( anIntValue ); //type inference fails

//however I can create a wrapper like this:
public static GenericDemo<T> Create<T> ( T value )
{
    return new GenericDemo<T> ( value );
}

//then this works - type inference on the method compiles
var item = Create( anIntValue );

Why doesn't C# support this class level generic type inference?

Answer 1

Actually, your question isn't bad. I've been toying with a generic programming language for last few years and although I've never come around to actually develop it (and probably never will), I've thought a lot about generic type inference and one of my top priorities has always been to allow the construction of classes without having to specify the generic type.

C# simply lacks the set of rules to make this possible. I think the developers never saw the neccesity to include this. Actually, the following code would be very near to your proposition and solve the problem. All C# needs is an added syntax support.

class Foo<T> {
    public Foo(T x) { … }
}

// Notice: non-generic class overload. Possible in C#!
class Foo {
    public static Foo<T> ctor<T>(T x) { return new Foo<T>(x); }
}

var x = Foo.ctor(42);

Since this code actually works, we've shown that the problem is not one of semantics but simply one of lacking support. I guess I have to take back my previous posting. ;-)